1. Field of the Invention
The present invention generally relates to a process for the manufacture of milk of lime.
2. Background Information
Milk of lime is generally manufactured by quenching burnt lime with a large excess of water. The term "burnt lime" is used to designate both burnt limes (calcium oxide, CaO) and burnt dolomite limes (calcium/magnesium oxide, CaO/MgO).
In special applications, lime which has been quenched to powder or dolomite lime in the form of a dry lime hydrate (calcium hydroxide, Ca(OH).sub.2) or dolomite lime hydrate (calcium/magnesium hydroxide, Mg(OH).sub.2 /Ca(OH).sub.2) can also be suspended in water. The lime hydrates manufactured by the conventional quenching processes, which are designated normal hydrates, typically have a characteristic specific surface of 15 to 22 m.sup.2 /g.
Milk of lime can be used, among other things, to clean gases and exhaust gases, e.g. according to the spray sorption principle, by spraying milk of lime into the current of exhaust gas, and then removing the dry residue by means of appropriate filters.
For certain applications, e.g. for cleaning gases and exhaust gases according to the spray sorption principle, milk of lime on the basis of hydrates which have a specific surface greater than 22 m.sup.2 /g can have significant advantages. Such hydrates with increased specific surface can be manufactured, for example, according to the process disclosed in German Patent No. 34 33 228, which corresponds to U.S. Pat. No. 4,636,379, or WO 92/09528. Alcohols such as methanol, ethanol, propanol and butanol, and ketone, ether and aldehyde, for example, can be used as organic solvents. Amines and glycols can be used as organic additives, e.g. ethylene glycol, diethylene glycol, triethylene glycol, monoethanolamine, diethanolamine, triethanolamine and mixtures of these products. The characteristic specific surface is typically greater than 30 m.sup.2 /g.
Compared to normal milk of lime manufactured from burnt lime, milk of lime manufactured by suspending lime hydrate which has a large specific surface in water, can reduce the lime consumption in the spray sorption process, e.g. as used in a waste incinerator, by about 40% while still achieving essentially the same cleaning action. There is also about 30% less residue from the cleaning of the exhaust gas. The use of normal milk of lime can present significant problems in conventional exhaust gas cleaning systems due to the stoichiometric ratios which must be maintained. The use of milk of lime manufactured from hydrate which has a large specific surface can make it possible to comply with the environmental requirements for waste gas cleaning essentially without any problem, for a normal composition of the exhaust gas.
One disadvantage which has been identified is that during the suspension of these lime hydrates which have been quenched with the addition of organic solvents and/or organic additives, a stable foam can be formed, which foam can lead to problems in the equipment which is part of the gas cleaning system.